Three decades after the emergence of HIV there is still no vaccine, and AIDS remains a threat to global public health. However, some HIV-infected individuals eventually develop broadly neutralizing antibodies (bNAbs), i.e., antibodies that neutralize a large panel of HIV viruses and that can delay viral rebound in HIV patients. Such antibodies are relevant to vaccine development, as evidenced by the prevention of infection observed after passive transfer to macaques. Antibodies obtained by recent methods target several epitopes on the viral spike gp120 protein. These antibodies show broad and potent activity, and are referred to as highly active agonistic anti-CD4 binding site antibodies (HAADs). HAADS mimic binding of the host receptor CD4 protein by exposing the co-receptor binding site on gp120. Despite isolation from different donors, HAADs are derived from two closely-related Ig VH genes that share gp120 contact residues (Sheid et al., 2011, Science, 333:1633-1637 and Zhou et al., Science, 2010, 329: 811-817.)
Structural analysis of gp120 complexed with VRC01 (a highly potent and broad HAAD), and gp120 complexed with each of VRC03 and VRC-PG04, (two new CD4bs antibodies sharing the VRC01 germline VH gene) revealed convergence of gp120 recognition despite low sequence identities (48-57% in VH; 62-65% in VL) (Wu et al, 2011, Science, 333:1593-1602). However, sequence differences between these clonally-unrelated anti-CD4 antibodies make it difficult to determine the structural features that yield neutralization potency and breadth to thereby obtain a potent HIV antibody that is effective across many HIV strains.